Molecular characteristics of astaxanthin and β-carotene in the phospholipid monolayer and their distributions in the phospholipid bilayer

Abstract

The molecular characteristics of the monolayers of astaxanthin with polar group on the β-ionone ring in the molecule and β-carotene without polar group and their interactions in mixed carotenoid–phospholipid monolayers and the effects of carotenoids on the phase behavior of the phospholipid bilayers were examined by the monolayer technique and differential scanning calorimetry (DSC). We found from the monolayer study that β-carotene had an amphiphilic nature. The molecular assembly of astaxanthin in the monolayer at the hydrophobic/hydrophilic interface was more stable than that of β-carotene. Dimyristoylphosphatidylcholine (DMPC) in the monolayer was miscible with astaxanthin in the range of 0–0.4 mol fractions of astaxanthin, but not fully miscible with β-carotene even at low concentrations below 0.1 mol fraction of β-carotene. Surface potential and compression/expansion cycles of β-carotene monolayer indicated the formation of molecular aggregates by itself. DSC study showed that when small amount of astaxanthin was added, the transition temperature of dipalmitoylphosphatidylcholine (DPPC) was markedly shifted to lower temperatures and that the transition peak was asymmetrically broadened, indicative of a significant depression in cooperativity of the gel to liquid–crystalline transition. The asymmetric DSC endothermic bands of DPPC incorporating small amounts of astaxanthin were well fit by deconvolution into two to three domains containing different concentrations of astaxanthin. On the contrary, the incorporation of β-carotene resulted in a small depression of the main transition temperature with a slight broadening of the transition peak, suggesting a small miscibility of β-carotene with the phospholipid bilayer or a formation of aggregates of β-carotene in the membranes. These results suggest that there would be a high localized concentration in the phase separated membrane for astaxanthin or β-carotene to function effectively as scavenger.